Electrostatic comb drive devices are utilized to provide motion in microelectromechanical systems (MEMS) devices. Such drive devices are employed, for example, in the fabrication of MEMS-type accelerometers, gyroscopes, and inertia sensing devices where rapid actuation is often necessary to effectively detect and measure motion and/or acceleration, or in the design or where size and/or weight are important design considerations. In the design of navigational and communications systems, for example, such devices are useful in measuring and/or detecting slight variations in linear and rotational motion of an object traveling through space. Other applications employing drive systems such as Steered Agile Beam (STAB) modules may also use electrostatic comb drive devices to provide a more precise alignment and orientation control of structures such as MEMS micro-mirrors and/or lenses.
In a typical comb drive device, a proof mass is supported over an underlying support substrate using a number of suspension beams or springs. The proof mass typically includes a number of drive elements that can be used to electrostatically move the proof mass above the support substrate in a particular manner. In certain designs, for example, the drive elements can include a number of interdigitated comb fingers spaced apart from each other by a relatively small gap (e.g. 1 to 2 microns), forming an overlapping region between adjacent comb fingers. During operation, an electrical charge can be applied to the comb fingers to induce an electrostatic charge between each overlapping region, converting electrical energy into mechanical energy. By varying the voltage signal applied, the proof mass can be configured to electrostatically oscillate back and forth in a desired manner, allowing one or more sense electrodes to measure up/down displacement of the proof mass induced by movement of the device about a rate axis.
In one illustrative embodiment, fabrication of electrostatic comb drive devices may begin with a silicon wafer substrate. In one embodiment, a boron-doped epitaxial (p++) layer is grown over the wafer substrate, which can then be patterned to form the desired microstructures using photolithography and etching techniques. The etched layer may be then bonded to an underlying support substrate (i.e. a “handle wafer”) using a suitable bonding process such as anodic bonding. The support substrate may include a number of mesas that support the proof mass and drive elements above the support substrate while allowing movement thereon. After bonding, the silicon substrate can be removed using a boron-selective etchant, leaving only the patterned p++ silicon mechanism bonded to the handle wafer. A number of suspension beams, springs, or other flexural elements are also typically used to constrain motion of the proof mass in a particular direction above the support substrate.
During fabrication, stresses induced between the epitaxial layer and wafer substrate, as well as fabrication imperfections, can result in imperfect alignment of the comb fingers, flexural elements, as well as other components, causing the proof mass to oscillate back and forth in a non-ideal manner above the support substrate. In certain applications, for example, such stresses and fabrication imperfections can cause uniform disengagement or shifting of the movable comb fingers with respect to the stationary comb fingers. In some applications, such stresses and fabrication imperfections can also cause the comb fingers to curve or bow slightly, resulting in a non-uniform disengagement of the comb fingers. During actuation, such shifting and/or bowing can induce electric fields both in the plane and perpendicular to the drive axis of the proof mass. As a result, an undesired out-of-plane electrical and/or mechanical component is introduced into the drive system, causing errors in the output signal that can reduce the ability of the device to measure subtle changes in movement.